If a disc is produced with a serial number sticker on it, as in on USDGC Rocs, is it legal to be thrown in a PDGA event? I understand that if a person adds their own sticker or a shop adds one, that it is not legal. But since it is an identifying mark by the manufacturer, is it an exception that is o.k. to use in a PDGA tourney? Or would someone have to take the serial number sticker off to use it?

This would be an extremely technical interpretation, but if someone was attempting to stroke me for the sticker being an identifiable mark (***edit: after a little more thinking, I would first say that a sticker doesn't satisfy the definition of being an identifying mark, because it isn't ink or pigment based) on the disc, I would tell them that the only kinds of identifying marks that are regulated under the rules are ink and pigment-based ones. Since stickers are neither of those things, they aren't regulated. A sticker by the manufacturer is clearly allowed.

802.01.F All discs used in play, except mini marker discs, must be uniquely marked in ink or pigment-based marking which has no detectable thickness"

Also, after having gone back and read the rules about discs used in play, as long as the sticker is placed on the underside of the flightplate and has no flaps or anything (I would argue this location doesn't alter the original flight characteristics) would also be legal, regardless of who put it there. There is a caveat that you could put enough stickers to make the disc way significantly more, which would altar the flight characteristics, which would be against the rules. (Could also put the disc outside the technical standards by going over the mold's max weight)

Stickers applied by the manufacturer do not need to be removed before the disc is used in sanctioned play. Here are the passages in the tech standards updated December 2009:

After the manufacturer is notified of PDGA approval and certification, the equipment may be used in PDGA-sanctioned competition. As of July 1, 2010, all newly-approved discs must carry the approved model name either engraved in the mold or as part of the disc art, permanent sticker label or by indelible ink marking. The full model name is recommended, but the use of unique abbreviations or codes is acceptable. Such codes must either be identified with the submission for approval, or supplied to the Technical Standards Committee for variants produced after the original approval.

It is also highly recommended that all approved discs carry the words "PDGA Approved" either engraved in the mold and/or as part of the disc art or a permanent sticker. Manufacturers are also encouraged to add the model and approval identification to all previously-approved discs when it is feasible to do so. All approved discs manufactured after January 1, 2012 must comply with these standards.